1. Field of the Invention
The present invention relates to single beam spectrophotometers and, more particularly, to the measurement of a sample in a single-beam spectrophotometer at one or more different light wavelength settings.
2. Description of the Prior Art
In a spectrophotometer a sample to be measured is positioned in a light beam and light transmitted, scattered, or otherwise passed or radiated by the sample is directed to a light detector, such as a multiplier phototube, which generates an output current signal proportional to the intensity of the detected light. It is often desired to measure the sample at different light wavelengths and, for that purpose, a wavelength dispersing mechanism, such as a monochromator, is positioned in the light path to select and control the wavelengths of light reaching the detector. The monochromator is adjustable to and through numerous individual wavelength settings across the light spectrum. At each setting the monochromator passes light at only a corresponding selected wavelength(s).
Sample measurements at different wavelength settings are complicated by the fact that a spectrophotometer's system energy varies as a function of wavelength. In other words, the detector output signal for 100% transmittance is different at one wavelength than the corresponding 100% transmittance output signal at another wavelength. So called double-beam spectrophotometers solve the wavelength dependent system energy problem by splitting the light beam into sample and reference beams, by sensing the energy change in the reference beam as the monochromator scans different wavelength settings, and by constantly adjusting dynode voltage of the photomultiplier detector to maintain a constant detector output signal for the reference beam changes. In this manner the output signal is maintained at a constant corrected level as the various wavelength settings are scanned. Unfortunately, such constant, repetitive dynode voltage correction introduces undesired output signal noise because of the many small corrections of detector voltage. In addition, by splitting the light beam, a double-beam system inherently reduces the energy remaining for the sample beam thereby magnifying the effect of the output signal noise.
Single beam instruments, on the other hand, are limited in ability to measure a sample across a wavelength scan range due to the fact that only a single dynode voltage is employed for the photomultiplier detector. To reduce detector output signal noise, the dynode voltage level is chosen to give a maximum or optimum output signal level. But since the detector inherently exhibits a limited dynamic range, the optimum dynode voltage, though optimum at one or some wavelengths, may cause the detector to saturate at other wavelengths. Moreover, single beam spectrophotometers require operator intervention to calibrate or set system gain at an optimum value prior for each wavelength setting at which a sample is to be measured and prior to the sample measurement at each setting. Moreover, a single beam system, by its single beam nature alone, measures a sample and a reference at different times in the same single beam, and operator intervention is required to make the two required measurements at each wavelength setting.